Creating 1D and 2D arrays

An array is a collection of elements of the same data type stored in consecutive memory locations and accessed by an index that starts from 0.

One dimensional array

Syntax: data_type name[size];

It is created by giving the data type, name and size. It can be initialised at the time of declaration.

int marks[5];                     /* 5 integers, garbage values */
int a[5] = {10, 20, 30, 40, 50};  /* initialised */

a[0] is 10 and a[4] is 50. The compiler reserves 5 × sizeof(int) bytes.

Two dimensional array

Syntax: data_type name[rows][columns];

It is a table (matrix) of rows and columns, stored row by row in memory. Two indices are needed to reach an element.

int b[2][3] = {{1, 2, 3}, {4, 5, 6}};  /* 2 rows, 3 columns */

b[1][2] is 6. The array holds 2 × 3 = 6 elements.

Working example

#include <stdio.h>

int main(void)
{
    int a[5] = {10, 20, 30, 40, 50};          /* 1D, initialised */
    int b[2][3] = {{1, 2, 3}, {4, 5, 6}};     /* 2D: 2 rows, 3 columns */
    int c[3];                                 /* 1D, values given at run time */
    for (int i = 0; i < 3; i++)
        c[i] = i * i;

    printf("a[2] = %d, b[1][2] = %d, c[2] = %d\n", a[2], b[1][2], c[2]);
    return 0;
}

Output:

a[2] = 30, b[1][2] = 6, c[2] = 4